Secondary-side trace inductances

There is also a major issue concerning secondary-side trace inductances, one that we will discuss a little later. Other than that, there are no issues, except of course the fact that because there is only one capacitor, the effective ESR won t be very good (nor the RMS ripple current-handing capability). 

Secondary Side Trace Inductances and Their Impact on Efficiency... 

If we want to mathematically estimate the inductance of the secondary-side traces, the rule-of-thumb we can use is 20 nH per inch. But here, we need to include the, full electrical path of the high-frequency output current — starting from one end of the secondary winding, returning to its other end, through the diode and output capacitor ). We will be surprised to calculate or measure, that even an inch or two of trace length can dramatically decrease the efficiency by 5 to 10% in low output voltage applications. 

This is also true because any trace inductance here gets multiplied by the square of the turns ratio, and reflects into the primary side, as discussed previously. This greatly increases the dissipation in the primary-side RCD/zener clamp and severely degrades the converter efficiency. We have to really struggle to minimize secondary-side inductances, especially for low output voltage rails, that is, those with higher turns ratios. 

In high-power offline Flybacks, the trace inductances on the secondary side reflect on to the primary side, and can greatly increase the effective primary-side leakage inductance and degrade the efficiency. The situation gets worse when we have to stack several output capacitors in parallel, just to handle the higher RMS currents. Long traces seem inevitable here. This has been discussed in detail previously. 

The secondary side leakage (uncoupled) inductance is associated not only with the actual transformer windings, but the lead-out terminations and even the PCB traces leading to and... 

The most frequently applied analytical methods used for characterizing bulk and layered systems (wafers and layers for microelectronics see the example in the schematic on the right-hand side) are summarized in Figure 9.4. Besides mass spectrometric techniques there are a multitude of alternative powerful analytical techniques for characterizing such multi-layered systems. The analytical methods used for determining trace and ultratrace elements in, for example, high purity materials for microelectronic applications include AAS (atomic absorption spectrometry), XRF (X-ray fluorescence analysis), ICP-OES (optical emission spectroscopy with inductively coupled plasma), NAA (neutron activation analysis) and others. For the characterization of layered systems or for the determination of surface contamination, XPS (X-ray photon electron spectroscopy), SEM-EDX (secondary electron microscopy combined with energy disperse X-ray analysis) and... 

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